1. Field of the Invention
This invention pertains to motor vehicles. In a preferred embodiment, it pertains to motor vehicles having both a two wheel drive configuration and a four wheel drive configuration and which are retrofitted for operation in an overdrive gear in two wheel drive. A reciprocal lockout prevents concurrent operation of the vehicle in overdrive and four wheel drive. The overdrive is arranged to reduce the magnitude of cyclic loading in single cardan joints connected to a propeller shaft of the vehicle.
2. Review of the Prior Art
Off road driving is becoming increasingly popular as evidenced by the increase in sales of motor vehicles which are capable of either two wheel drive or four wheel drive modes of operation. In this time of diminishing resources, improvements in fuel economy of motor vehicles, which can be realized by an overdrive arrangement, are continually being sought to optimize performance.
The present invention is believed to have particular significance when used in a motor vehicle capable of operation in either four wheel drive or two wheel drive, but the invention can be used to advantage in more conventional motor vehicles which have only two wheel drive capability.
Motor vehicles transmit power from the engine to a drive axle. In many vehicles having both two and four wheel drive capability, such as a Jeep CJ-5 or an International Harvestor vehicle, in which this invention has particular significance, tubular propeller shafts transmit engine torque from a transfer case to the front and rear axle; the transfer case houses the gearing which enables selection between two wheel and four wheel vehicle operation modes. Single cardan universal joints connect each shaft to relevant transfer case and axle yokes. A substantially horizontal drive shaft driven by the engine is connected to the transfer case yoke, and an input shaft of a drive axle differential is connected to the axle yoke. The drive shaft and the input shaft are generally parallel but are not coaxial so that the drive shaft is high in the vehicle relative to the differential. This means that the propeller shaft is inclined relative to a substantially horizontal reference plane associated with the vehicle chassis and to the drive and input shafts with which it is coupled. It also means that the cardan joints operate at an angle.
Neither single cardan joint is a constant velocity universal joint. When torque is transmitted through single cardan universal joints operating at an angle, the rotating speeds of the joint driving and driven yokes will differ from each other during a single rotation of the driving yoke. In operation, the driving yoke rotates at a constant speed while the driven yoke speeds up and slows down twice every revolution.
This variation in driven yoke rotating speed is proportional to the operating angle of the universal joint. In effect, the greater the universal joint operating angle, the greater the speed fluctuation of the driven yoke. Speed fluctuation causes the cardan joints to experience substantial cyclic stresses during each rotation of the propeller shaft. Cyclic stresses tend to decrease the useful life of the cardan joints and the propeller shaft by causing progressive degradation, driveline vibration and eventual failure. When this occurs, it becomes necessary to repair or replace the cardan joints or the propeller shaft, which requires servicing the vehicle.
An overdrive arrangement permits a vehicle to operate at highway speeds with a lower engine rpm and greater fuel economy than would be possible in direct drive, i.e., without the overdrive capability. A higher gearing ratio is used in overdrive than in direct drive. In vehicles having four wheel drive capability, such as the Jeep CJ-5, the gearing for shifting between two wheel and four wheel drive is contained in the transfer case located at the rear of the transmission. To modify such a vehicle for overdrive, it is highly desirable to retrofit the transfer case so as to take advantage of the existing vehicle gearing and shifting mechanisms to the greatest extent possible. Since the front end of the transfer case is attached to the vehicle transmission, an ideal place to install the overdrive would be where the transfer case rear drive shaft connects to the rear inclined propeller shaft, i.e., at the back of the transfer case. While this location would take advantage of the existing gearing and shifting mechanisms of the vehicle, it effectively decreases the length of the rear propeller shaft because an overdrive mechanism is inserted between the transfer case and the rear differential. Any modification to provide an increased gearing ratio which effectively decreases the length of the rear propeller shaft, without adjusting its angularity relative to the rear drive axle, will aggravate a problem which is already troublesome in the Jeep CJ-5 vehicle.
A need exists for an overdrive mechanism connectible to a transfer case or transmission of a vehicle having an inclined driving propeller shaft and which does not produce an increase in that inclination.
Since the overdrive feature is useful only at highway speeds, where fuel economy rather than power or engine torque is the primary consideration, it is not necessary to adapt the overdrive mechanism for operation while the vehicle is engaged in the four wheel drive mode. By attaching the overdrive mechanism to the back of the transfer case, only the rear drive axle is affected. However, this presents another problem. If an overdrive gear were to be engaged while the vehicle is being operated in four wheel drive, the front axle would turn at a different rate of speed from the rear axle. This would create a host of manifest mechanical problems.
A need exists for an overdrive arrangement which prevents concurrent operation of the vehicle in the overdrive gear and in four wheel drive mode.
The use of this invention in a motor vehicle provides a simple and effective way to provide an overdrive arrangement which reduces cyclic loading stresses on single cardan joints operating at an angle and, in its preferred form, when used in a motor vehicle having both two wheel drive and four wheel drive modes, provides a reliable way to prevent simultaneous operation of the vehicle in four wheel drive and overdrive modes.
The present invention provides other advantages which are set forth in the following detailed description of preferred embodiments thereof.